Which of these numbers is prime? ${3,\ 9,\ 15,\ 64,\ 85}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 9 are 1, 3, and 9. The factors of 15 are 1, 3, 5, and 15. The factors of 64 are 1, 2, 4, 8, 16, 32, and 64. The factors of 85 are 1, 5, 17, and 85. Thus, 3 is a prime number.